Implantable medical devices for producing a therapeutic result in a patient are well known. An example of such an implantable medical device includes implantable neurostimulators used for the treatment of movement disorders such as Parkinson's Disease, essential tremor, and dystonia. Other examples of such implantable medical devices include implantable drug infusion pumps, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators, and cochlear implants. Of course, it is recognized that other implantable medical devices are envisioned that utilize energy delivered or transferred from an external device.
A common element in all of these implantable medical devices is the need for electrical power in the implanted medical device. The implanted medical device requires electrical power to perform its therapeutic function whether it be driving an electrical infusion pump, providing an electrical neurostimulation pulse, or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.
Typically, a power source for an implantable medical device can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. Having electrical wires that perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power for therapy is, at least, a large inconvenience. The second form utilizes single cell batteries as the source of energy of the implantable medical device. This can be effective for low power applications, such as pacing devices. However, such single cell batteries usually do not supply the lasting power required to perform new therapies in newer implantable medical devices. In some cases, such as an implantable artificial heart, a single cell battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all or nearly all of its energy in less than a year. This is not desirable due to the need to explant and re-implant the implantable medical device or a portion of the device. One solution is for electrical power to be transcutaneously transferred through the use of inductive coupling. Such electrical power or energy can optionally be stored in a rechargeable battery. In this form, an internal power source, such as a battery, can be used for direct electrical power to the implanted medical device. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external power source temporarily positioned on the surface of the skin. Several systems and methods have been used for transcutaneously inductively recharging a rechargeable battery used in an implantable medical device.
Transcutaneous energy transfer through the use of inductive coupling involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. The internal coil, or secondary coil, is part of or otherwise electrically associated with the implanted medical device. The external coil, or primary coil, is associated with the external power source or external charger or recharger. The primary coil is driven with an alternating current. A current is induced in the secondary coil through inductive coupling. This current can then be used to power the implanted medical device or to charge or recharge an internal power source or a combination of the two.
For implanted medical devices, the efficiency at which energy is transcutaneously transferred may be crucial. First, the inductive coupling, while inductively inducing a current in the secondary coil, also has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. Since heating of surrounding tissue is limited, so also is the amount of energy transfer that can be accomplished per unit time. The higher the efficiency of energy transfer, the more energy can be transferred while at the same time limiting the heating of surrounding components and tissue. Second, it is desirable to limit the amount of time required to achieve a desired charge, or recharge, of an internal power source. While charging or recharging is occurring, the patient necessarily has an external encumbrance attached to his or her body. This attachment may impair the patient's mobility and limit the patient's comfort. The higher the efficiency of the energy transfer system, the faster the desired charging or recharging can be accomplished thus limiting any inconvenience to the patient. Third, the amount of charging or recharging can be limited by the amount of time required for charging or recharging. Since the patient is typically inconvenienced during such charging or recharging, there is a practical limit on the amount of time during which charging or recharging should occur. Hence, the size of the internal power source can be effectively limited by the amount of energy that can be transferred within the amount of charging time. The higher the efficiency of the energy transfer system, the greater amount of energy that can be transferred and, hence, the greater the practical size of the internal power source. This allows the use of implantable medical devices having higher power use requirements and providing greater therapeutic advantage to the patient and/or extends the time between charging effectively increasing patient comfort.
Implantable medical devices, external power sources, systems and methods have not always provided the best possible system or method for allowing the patient to be ambulatory during energy transfer and/or charging. Physical limitations related to the energy transfer and/or charging apparatus and methods as well as necessary efficiencies of operation can effectively limit the patient's ability to move around during such energy transfer and/or charging and can deleteriously affect patient comfort.